In a reflector antenna[?], the relative reduction in the effective aperture of the antenna caused by the masking effect of other antenna parts, such as a feed horn or a secondary reflector, which parts obstruct the radiation path.
... adopted a Book of Confessions[?]. The BOC contains the Nicene Creed, Apostle's Creed[?], Scots Confession[?], Heidelberg Catechism[?], Second Helvetic Confession[?], ...